The message from the headmaster of an independent school announcing a teacher's imminent retirement appeared, at first glance, to be innocent enough --- even if the second sentence did start a little clumsily.

"It is with mixed feelings that I announce the retirement of Mr Roger Clark at the end of this term," wrote Mark Dunning in the newsletter of Orley Farm School in Harrow, north-west London.

"We all now know every really great teacher has to finish one day and Mr Clark will do so at the end of this term."

While parents of pupils at the £13,000-a-year school may not have given the message a second glance, Mr Clark, who taught French and English, was quick to spot how the head had used code to express his true feelings towards him.

He noticed that the first letter of the first six words in the second sentence spelt out a message known as an acrostic, which could hardly have been more insulting.